This invention relates in general to welding and, more particularly, to a process for welding high and low carbon steels together and to a weldment formed by the process.
If steel is heated above its upper critical temperature, which varies depending on the amount of carbon in the steel, the steel assumes a phase known as austenite, which is a solid solution of iron and carbon. Should the steel undergo a rapid cooling, some of the austenite will transform into martensite, which is extremely hard, while much of the rest will remain as retained austenite, which is considerably softer and more ductile, although other ductile constituents will usually be present as well. The amount of martensite that is formed depends to a large measure on the amount of carbon dissolved in the austenite at the higher temperature. High carbon steels contain harder martensite than low carbon steels. Being harder, high carbon martensite steel resists wear and deformation and is therefore useful in bearings, gears, and products for like applications. But that very same steel lacks ductility, that is to say, it is brittle and tends to crack. Low carbon (ferite, hamite) steel, when quenched, contains low carbon martensite and some Nonmartensitic Transformation Products (NMTP). The low carbon quenched steel is not as brittle, but then it does not resist wear and deformation as well.
A case carburized part, such as a bearing race, possesses the beneficial characteristics of both high and low carbon steel. It has a ductile core that can withstand shocks and a hard case that withstands deformation and wear.
Joining materials by welding can occur either by melting the materials in the vicinity of the joint or by avoiding melting and creating a solid-state diffusion bond. If melting occurs during the welding process, that process is called fusion welding. Consider the situation of fusion welding a component made from two steels. When solidification begins, crystals of austenite form on the unmelted surfaces and grow in size and quantity as solidification progresses. The carbon concentration of the solid must be less than that of the liquid from which it formed, so excess carbon remains in the liquid. The liquid is enriched with carbon. As the temperature continues to decrease, the melted zone becomes mostly solid with a diminishing volume of liquid remaining around the grain.
As the last liquid freezes to solid, that freshly formed solid material shrinks in volume. The solid phase has a smaller volume. The shrinkage creates residual stresses within the fresh solid, know as a melt zone. With continued cooling, most of the higher temperature austenite phase transforms to ferrite, pearlite, bainite, and/or martensite depending upon the carbon concentration and the cooling rate.
The cooling rate can be affected by a heat treatment immediately prior to welding that raises the temperatures of the two components. This elevated temperature causes the cooling rate of the solidified melt zone to be retarded, thereby, allowing NMTP to be formed. The NMTP are resistant to cracking. Microstructures containing large volume fractions of martensite are not resistant to cracking. Thus, the solidified and cooled melt zone does not crack in response to the shrinkage-induced residual stress because of the minimization of martensite formation. Preheating the components is a well-known practice, but preheating may not be possible. Softening of the components, dimensional change, distortion, and/or undesirable scaling and tinting of the surface may render preheating undesirable.
Consider the situation of fusion welding steels without a preheat. The mass of metal in the components functions as a heat sink and rapidly cools the steel in the region of the weld—self quenching in effect—and as a consequence, the steel in the melt zone acquires a good measure of martensite. The solidified cooled melt zone formed during the welding of low carbon steels without a preheat consists of the relatively soft low carbon martensite, some NMTP, and some retained austenite. The melt zone does not crack in response to the shrinkage-induced residual stresses.
Joining of two high carbon steels without a preheat presents a special problem for the welder. The solidified and cooled melt zone of high carbon steel consists of relatively hard high carbon martensite and a lesser amount of retained austenite. This brittle microstructure cracks in response to the shrinkage-induced residual stresses. Cracks within the melt zone caused by shrinkage-induced residual stresses are known as “solidification cracks” and “hot cracks”.
The foregoing has focused upon the melt zone. Now, consider the situation of the heat affected zone (HAZ) adjacent to the melt zone. The heat of welding raises its temperature above the upper critical temperature as well. As the HAZ cools, it is also subject to shrinkage-induced residual stresses. However, the material in the HAZ remains cooler and stronger than the hotter melt zone. Cracking of the HAZ does not necessarily accompany solidification. If it occurs, it will occur after a delay ranging from seconds to days.
Fusion welding of two low carbon steels results in a HAZ that is first austenitized and then cooled to form the crack-resistant composite microstructure containing low carbon martensite, NMTP, and some retained austenite. Fusion welding of two high carbon steels results in a heated and cooled HAZ creating the crack-susceptible high carbon martensite and some retained austenite. Thus, the HAZs of high carbon steels are prone to cracking.
Not all weldments contain fusion welds—the welds can be diffusion bonds. An example is friction stir welding. If there is no melting, a melt zone formation and consequent shrinkage stresses fail to develop. Although there is no melt zone, a HAZ is created on both sides of the joint. The HAZ microstructure that develops is dependent upon cooling rate and carbon concentration. In the absence of a preheat, the cooling rate will be fast due to the self-quenching. The HAZ microstructure will always contain martensite because of the rapid quench. The carbon concentration is then the determining factor for HAZ microstructure. Solid state welding of low carbon steels will create crack-resistant microstructures, and crack-susceptible microstructures will be produced in HAZs of high carbon steels.
When a welder fusion welds a low carbon steel to a high carbon steel with no filler metal, a somewhat similar problem develops. Again a melt and a HAZ develop and undergo a self quench. The steel in the melt zone represents a mixture of high and low carbon steels, and as a consequence has a carbon content intermediate that of the two steels. Usually, it is not enough to create hard plate-type martensite, so the melt zone remains relatively ductile. That much of the HAZ that lies with the low carbon steel is not sufficiently brittle to cause concern. However, the remainder of the HAZ, that is the portion that lies within the high carbon steel, acquires considerable plate-type martensite and as a consequence is hard and brittle and subject to cracking under stresses, both residual and applied. The problems with welding high carbon steel, either to more high carbon steel or to a low carbon steel, are particularly acute with butt welding and fillet welding. But they appear in the laser lap seam welding and resistance welding as well.
Typically, the race of an antifriction bearing is formed from a case carburized steel that has undergone a heat treatment to produce a hard surface on the race, or else it is formed from a high carbon steel that is through hardened in a heat treatment. But often a race must be fitted with a shield or some other component, often a stamping formed from low carbon steel. Since welding is not a viable option under current technology, the component is pressed over, snapped into or onto, or in some other way mechanically connected to the race. Welding would serve as a desirable alternative if practical. To be sure, procedures exist for lessening the deleterious results from welding high carbon steel to low carbon steel. One is preheating. However, that softens both steels, perhaps more than desired. Another resides in applying temper pulses to the weld after it is made. These, however, do not produce the desired ductility. Normally, they lower the hardness to no less than about 58 HRC (Rockwell C) when preferably it should be less than 50 HRC. Then again there is traditional tempering, but it is a diffusion process that requires considerable time and still lowers the hardness of the high carbon steel to only about 58 HRC.